Up to now, a technique has been known in which each phase current is detected on the basis of a shunt voltage which is a voltage between both ends of a shunt resistor disposed on a bus of an inverter. For example, in JP-A-2013-17363, a current detection value of a drift current detected in a state where a motor current is assumed to be zero is set as a current offset value, and a current detection value of the motor current is compensated according to the offset value.
The shunt voltage of the shunt resistor disposed on the bus of the inverter may be zero even in a normal state. For that reason, for example, even if abnormality that the shunt voltage is fixed to zero due to short circuit of the shunt resistor occurs, it cannot be discriminated whether the shunt voltage is fixed to zero, or not, in JP-A-2013-17363 corresponding to US 2013/0009587.